The present invention relates to boost converters and more particularly to continuous variable frequency boost converters.
Many conventional off line power supplies have input stages that contain a bridge rectifier followed by a large bulk capacitor. Such power supplies only draw current from the peak of the input line voltage waveform, when the input voltage is above the bulk voltage of the capacitor. The power factor of power supplies with bulk capacitors is low, reducing the amount of power that can be drawn from a source of a given volt ampere (VA) rating. The utility voltage waveforms are distorted by the excessive current drawn at the peak of the sinusoidal wave.
Power factor corrected power supplies place a circuit having a series inductor before the bulk capacitor. Usually this is a switching power supply circuit, often of the boost type. When a constant frequency, continuous conduction mode boost converter is used, input current is sensed with a resistor, and duty cycle is adjusted to force the input current waveform to match the shape of the input voltage wave. The amplitude of the current is adjusted at a very slow rate to keep the voltage on the bulk capacitor approximately constant. Efficiency of this type of circuit is reduced by the power dissipated in the sense resistor, switching losses in the power switch, and reverse recovery current of the diode.
Switching the power switch of the boost converter when the inductor current is at a minimum, minimizes switching losses in the transistor, transition losses, reverse recovery losses through the diode between boost inductor and output capacitor, and common-mode noise. The current through the output diode of the boost converter or the voltage across the output diode is typically monitored to determine when inductor current is at a minimum. Sensing the current in the diode or the voltage across the diode introduces errors due to inductor ringing due to interaction of the inductance with circuit capacitances making accurate detection of zero current in the inductor difficult. Inaccurate detection of zero current results in transistor switching with increased diode recovery losses and transistor switching losses.
To reduce losses, the boost converter can be operated in a critically continuous mode which occurs at the boundary between the continuous and discontinuous mode.
It is an object of the present invention to provide a critically continuous boost converter with accurate determination of zero current in the boost inductor without measuring inductor current.
It is another object of the present invention to provide a critically continuous boost converter which achieves near unity power factor.
It is still another object of the present invention to provide a critically continuous boost converter with reduced rectifying diode recovery loss and reduced switch capacitance loss.
It is a further object of the present invention to reduce crossover distortion in the current waveforms drawn by variable frequency, critically continuous high power factor switching power supplies.